Is sleeping on stomach bad for someone's sciatica?

Yes: Sleeping on stomach causes back extension which increases stress on facet joints and closes foramen this can put more pressure on nerve roots and increase back and leg pain.

Position of comfort: Lying on the back with pillows under the knees and/or lying on the side with a pillow between the knees are often the best positions when dealing with back pain or sciatica. Unfortunately once you fall asleep you will have no control over how you are positioned.

Can Be Bad: Usually stomach sleeping is not helpful for sciatica pain. Recommend sleeping on side with 1-2 pillows between legs or sleeping on back with legs raised up with 2-3 pillows under them.